Hot melt adhesives are 100% solid materials which do not contain or require any solvents and are solid at room temperature but, on application of heat, melt to a liquid or fluid state in which form they are applied to a substrate. On cooling, the adhesive regains its solid form and gains its cohesive strength. Hot melt adhesives which are applied in molten form, cool to solidify and subsequently cure by a chemical crosslinking reaction have been prepared using specific materials such as polyurethanes.
Reactive hot melts are one-component, 100% solid, solvent-free urethane prepolymers. Unlike conventional hot melts that can be repeatedly heated from solid state and flowed to a liquid form, the reactive hot melt behaves as a thermoset and goes through an irreversible chemical reaction once dispensed in the presence of ambient moisture. The reactive hot melts are isocyanate terminated prepolymers that react with surface or ambient moisture in order to chain-extend, forming a new polyurethane polymer which offers performance superior to that obtained with conventional hot melt.
It is known from U.S. Pat. No. 5,021,507 to Stanley et al. that the incorporation into otherwise conventional polyurethane hot melt adhesives of low molecular weight polymers formed from ethylenically unsaturated monomers containing no reactive hydrogen provides an improvement with respect to cohesive and adhesive strength and assists in forming bonds with some of the more difficult to adhere substrates. In order to incorporate the low molecular weight polymer into the polyurethane, it is possible to polymerize the respective monomers within the urethane prepolymer or, optionally, to add an already polymerized low molecular weight polymer into the urethane prepolymer. Typical monomers useful include acrylic monomers such as the C1 to C12 esters of acrylic or methacrylic acid as well as ethylenically unsaturated monomers containing moisture reactive functional groups such as silane or reactive isocyanate. Typical low molecular weight polymers are represented by Elvacite 2013, a 64% butylmethacrylate/36% methylmethacrylate copolymer having an I.V. of 0.2.
While the class of adhesives described in the aforementioned patent has substantially improved properties for most applications, the fact that the low molecular weight polymer is merely blended into, and not chemically bound, within the urethane still creates problems in areas where high heat and/or solvent resistance is required such as for automotive adhesive and sealant applications. Moreover, the range of acrylic monomers useful is somewhat limited in terms of Tg. Thus the higher Tg, more polar acrylics are found to be more compatible with the urethanes while the lower Tg, non-polar acrylics could not be as readily incorporated into the system. There is, however, a need for adhesives containing these lower Tg materials since they will provide for a more elastomeric adhesive and may improve the adhesion to certain metal substrates as well as provide for longer open time or bond range.